Photochromic materials undergo a structural transformation from one form (or state) to another in response to certain wavelengths of electromagnetic radiation, with each form having a characteristic absorption spectrum for visible radiation. For example, thermally reversible photochromic materials are capable of transforming from a ground-state form to an activated-state form in response to actinic radiation, and reverting back to the ground-state form in response to thermal energy and in the absence of the actinic radiation. As used herein, the term “actinic radiation” refers to electromagnetic radiation that is capable of causing a photochromic material to transform from one form or state to another.
Photochromic materials adapted for use in ophthalmic applications are usually essentially colorless or “optically clear” when not exposed to actinic radiation (i.e., in the ground-state form) and exhibit a visible color that is characteristic of the absorption spectrum of the activated-state form of the photochromic material upon exposure to actinic radiation. Photochromic compositions and articles that contain one or more photochromic materials, for example, photochromic lenses for eyewear applications, may display clear and colored states that generally correspond to the optically clear and colored states of the photochromic material(s) that they contain.
For certain applications, it is desirable that the photochromic material be able to make the transition from the colorless, ground-state form to the colored, activated-state form as quickly as possible. It is often additionally desirable that the photochromic material be able to make the reverse transition from the colored, activated-state form back to the colorless, ground-state form as quickly as possible. For example, in photochromic eyewear applications, ophthalmic lenses comprising photochromic materials may transform from a clear state to a colored state as the wearer moves from a region of low actinic radiation, such as, indoors, to a region of high actinic radiation, such as, exposed to sunlight. As the lenses become colored, less electromagnetic radiation having wavelengths within the visible and/or ultraviolet regions of the electromagnetic spectrum is transmitted through the lens to the wearer's eyes. In other words, more electromagnetic radiation is absorbed by the lenses in the colored state than in the clear state. When the wearer subsequently moves from the region of high actinic radiation back to a region of low actinic radiation, the photochromic material in the eyewear may revert from the colored, activated-state form to the optically clear, ground-state form in the absence of actinic radiation and absorbance of thermal energy. If the transition from the optically clear state to the colored state takes several minutes or more upon exposure to actinic radiation, the benefit of the reduced transmittance of visible and/or ultraviolet radiation that may be derived from the lenses in the colored state may be diminished. Further, if the transition from the colored state to the optically clear state takes several minutes or more once removed from actinic radiation, the wearer's vision may be less than optimal during this time due to the combined effects of the lower ambient light and the reduced transmission of visible light through the colored lenses. Accordingly, it would be desirable to develop photochromic materials that may more quickly transition from the optically clear, ground-state form to the colored, activated-state form, as well as from the colored, activated-state form to the optically clear, ground-state form.
Additionally, conventional photochromic materials often exhibit a “directional” dependency. That is, color change is most pronounced when the photochromic material is facing a light source directly, such as direct sunlight, with the photochromic effect being less noticeable or complete when the material is exposed indirectly to a light source. Accordingly, there is a need for photochromic materials that are less directionally dependent, demonstrating a more consistent degree of color change substantially independently of the orientation of the light source relative to the photochromic material.